Regeneration

by Thomas Hunt Morgan

The strongest evidence in favor of this view is found in the behavior of small pieces of an egg, or of a protozoon, or even of a many-celled organism. A lower limit of organization is very soon reached, below which the piece fails to produce the characteristic form, although all the necessary elements are present in the piece to produce the entire structure. The size of these pieces is enormously large as compared with the size of the cell, or of the imaginary elements of Nägeli, Weismann, Wiesner, etc. These results indicate that the organization is a comparatively large structure.

A few writers have either ignored the presence of smaller units, or have dealt with the organism from a purely chemical and physical point of view. They attempt to account for the changes in the organism as the outcome of known physical and chemical principles. It must, of course, be granted that in a sense the properties of the organism are the result of the material basis of the organism; but in another sense this idea gives a false conception of the phenomena of life, because, if we were simply to bring together those substances that we suppose to be present in the organism we have no reason to think that they would form an organism, or show the characteristic reactions of living things. Even from a chemical point of view we can see how this result could not be expected, for it is well known that the order in which a compound is built up, i.e. the way in which the atoms or molecules are introduced into the structure, is an important factor in the making of the compound. When we remember the immense period of time during which the organisms living at present have been forming, we can appreciate how futile it will be to attempt to explain the behavior of the organism from the little we know in regard to its chemical composition. Its chief properties are the result of its peculiar structure, or the way in which its elements are grouped. This structure has resulted from the vast number of influences to which the organism has been subjected, and while it may be granted that if we could artificially reproduce these conditions an organism having all the properties that we associate with living things would result, yet the problem appears to be so vastly complicated that few workers would have the courage to attempt to accomplish the feat of making artificially such a structure. To prevent misunderstanding, it may be added that while from the point of view here taken, we cannot hope to explain the behavior of the organism as the resultant of the substances that we obtain from it by chemical analysis (because the organism is not simply a mixture of these substances), yet we have no reason to suppose that the organism is anything more than the expression of its physical and chemical structure. The vital phenomena are different from the non-vital phenomena only in so far as the structure of the organism is different from the structure of any other group of substances.

Nägeli has stated that each part acts as though it knew what the other parts are doing. His idea of the idioplasm involves a conception of the organism as a whole and not simply the sum total of a number of parts. Hertwig, who maintained at one time that the development of the embryo is the resultant of the action of the cells on each other, admits in his work on Die Zelle und die Gewebe that while this is in part true, yet on the other hand the whole also exerts an influence on its parts. Driesch, who hypothetically suggested at one time that the nuclei act as centres of control of the cell by means of enzymes, has later adopted a widely different view. Whitman has made a strong argument to the effect that the cell theory is too narrow a standpoint from which to treat the organism, and on several occasions I have urged that the organism is not the sum total of the action and interaction of its cells, but has a structure of its own independent of that of the cells.

This discussion will suffice to show some of the opinions that have been held as to the nature of the organization of the organism. Let us next ask what properties we may ascribe to it.

It has been found that certain polar, or rather dimensional, relations are characteristic of the organization. The term “polarity” expresses this in a limited way, but refers only to one line having two directions, while we now know that the dimensional properties relate to the three dimensions of space, and for this idea we might make use of the term heterotropy. Thus we find that a piece of a bilateral animal regenerates a new anterior end from the part that lay nearer the anterior end of the original animal, a new right side from the part that was nearest the original right side, and a new dorsal part from the region that lay near the original dorsal part, etc.

The polarity of a part can be changed in certain forms, as in tubularia, by exposing the posterior cut-end to the external factors that bring about the formation of a hydranth, or, as in hydra, by grafting in a reversed direction a smaller piece on a larger one. In Planaria lugubris and in the earthworm the polarity of the new tissue may be reversed, as compared with that of the part from which it develops, if the new part arises from certain regions of the body. A curious instance of the effect of the polarity is shown by the regeneration from an oblique surface in planarians. The new head arises from the more anterior part of the new material, rather than from the middle of the anterior oblique surface, and the new tail arises from the more posterior part of the posterior oblique surface. As an analysis of this result has been already attempted in an earlier chapter, it will not be necessary to go further into this question here.

The development of a new part at right angles to an oblique surface has also been described, and it has been pointed out that the result appears to be due to the symmetrical development of the new structure in the new part. This symmetry of the newly forming part must be also counted as one of the properties of the organization.

Finally, the mode of regeneration of a new, bifurcated tail in the teleost, stenopus, shows that the new part may very early become moulded into the characteristic form, and that the growth of the different parts is regulated by the structure assumed at an early stage. The new part does not grow out at an equal rate until it reaches the level of the notch of the old tail, and then continue to grow at two points to produce the bilobed form of the tail; but the bilobed condition appears at the very beginning of the development.

These illustrations give us nearly all the data that we possess at present on which to build up a conception of the organization. That we must fail in large part fully to grasp its meaning from these meagre facts is self-evident. The main difficulty seems to lie in this,—that when we attempt to think out what the organization is we almost unavoidably think of it as a structure having the properties of a machine, and working in the way in which we are accustomed to think of machines as working. The most careful analysis of the “machine theory,” as applied to the phenomena of development and of regeneration, has been made by Driesch. It has been pointed out that in his Analytische Theorie Driesch assumed that development is due to “given” properties in the egg; that each stage is initiated by some substance contained in the egg acting on the stage that has just been completed. That is, each stage is the condition of the following. The “rhythm” of development is accounted for in this way. The changes are described as due to chemical processes (including also ferment actions). The nucleus is supposed to contain all the different kinds of ferments that act, when set free, as stimuli on the protoplasm; but since the ferments are always set free at the propitious moment, Driesch was obliged to assume that the cytoplasm acts on the nucleus in such a way as to make it produce the proper ferment for the next stage. Thus the cytoplasm first influences the nucleus, the latter sets free a specific ferment that starts a new chemical change in the cytoplasm, and the changed cytoplasm may then react again on the nucleus, and a different ferment be set free, etc. Each change is therefore not only an effect of what has gone before, but the cause of the next process.[132] Driesch points out that it is necessary at this stage to make a further assumption, because the cytoplasm must not only be acted upon by the ferment, but it must itself be of such a sort that it responds to the action. This leads to a great complication of the phenomena; but the assumption does not depart, in the last analysis, from the idea of the cell as a system in a mechanical sense. This assumption of a receiving and an answering station for the stimuli carries with it the further assumption of a many-sided “harmony.” Without a harmony at each step in the development there could be no orderly ontogeny. The assumption of this harmony introduces a new element into the series of hypotheses. The appearance of a causal explanation was given in those parts of the argument preceding the introduction of the assumption of a harmony, but with the admission of this new element into the argument, the causal point of view is left. Driesch says in this connection: “If we cannot gain a singleness of view in the way that has been followed, we can reach this in another way. Indeed, the way of doing so has been already implied in that part of the theory dealing with the harmony of the phenomena. The existence of this harmony is inferred, because, in the large majority of cases, the ontogeny leads to a typical result. Therefore we must assume that the conditions for the end result are given—the conditions are the harmony itself.” Put somewhat less obscurely, if more crudely, we may express Driesch’s idea by saying that the harmony that stands for a hen is given in the hen’s egg.

Driesch adds: “Because a typical result always follows, therefore every single step in the ontogeny must be judged, from an analytical standpoint, from the point of view of the result itself. The result is the purpose of the ontogeny. It is as though we visited daily a wharf where a ship is being built,—everything appears a chaos of single pieces, and we can only understand what we see when we consider what is to be made. Only from a teleological point of view can we speak of a development, for this term expresses the very existence of an object to be developed. The term is used fraudulently if it is intended to mean that the development is the outcome of ‘processes,’ using this term in the sense that a mountain or a delta develops from physical processes.” “We can only reach a satisfactory view of the phenomena when we introduce the word ‘purpose.’ This means that we must look upon the ontogeny as a process carried out in its order and quality as though guided by an intelligence. We arrive at this conclusion, because the individual whole is ‘given,’ as the clearly recognized goal of the entire process of development.”